1. Field of Invention
The invention relates to an active matrix array structure. More particularly, the invention relates to an active matrix array structure with an overcoat layer.
2. Description of Related Art
A thin film transistor liquid crystal display panel (TFT LCD panel) is constructed with an active matrix array structure, a color filter array structure and a liquid crystal layer. The active matrix array structure is configured with a plurality of arrays in rows and columns of active devices, i.e. the thin film transistors (TFT), and a pixel electrode corresponding to each thin film transistor. Each of the above thin film transistors includes a gate, a channel, a drain and a source, and each thin film transistor serves as a switch device of the liquid crystal display panel.
FIG. 1 is a top view of a conventional active matrix array structure having an overcoat layer. FIGS. 2A to 2E are schematic cross-sectional views of FIG. 1 along the cutting lines I-I′ in which selected manufacturing process steps of the conventional active matrix array structure are illustrated. Referring to FIGS. 1 and 2A, according to a conventional manufacturing process for an active matrix array structure, a first photomasking process is performed to form a gate 112 and a scan line 120 connected to the gate 112 on the substrate 50. A scan contact pad 122 is concurrently formed at the end of the scan line 120. Thereafter, a gate insulating layer 130 is formed on the substrate 50.
As shown in FIG. 1 and FIG. 2B, a second photomasking process is performed to form a channel region 114 on the gate insulating layer 130 above the gate 112. Continuing to FIGS. 1 and 2C, a third photomasking process is performed to form a source electrode 116, a drain electrode 118 and a data line 140 that connects to the source electrode 116. Another data contact pad 142 is concurrently formed at the end of the data line 140. The substrate 50 is then covered by a passivation layer 150.
Referring to FIGS. 1 and 2D, a fourth photomasking process is conducted to form a patterned overcoat layer 160 on the passivation layer 150. Furthermore, the drain electrode 118 and the passivation layer 150 above the contact pads 122, 142 are exposed. The patterned overcoat layer 160 serves as an etching mask for removing the gate insulating layer 130 and the passivation layer 150 above the contact pads 122, 142, and the passivation layer 150 above the drain electrode 118.
Ultimately, as shown in FIGS. 1 and 2E, a fifth photomasking process is performed to form a pixel electrode 170 on the patterned overcoat layer 160, and a patterned transparent conductive layer 172 on the surfaces of the contact pads 122, 142. In accordance to the above fabrication process, forming a patterned overcoat layer 160 on the passivation layer 150 raises the display aperture ratio of the liquid crystal display panel. The patterned overcoat layer 160 with a greater thickness can preclude a large parasitic capacitor from being generated between the pixel electrode 170 and the data line 140 to affect the characteristics of the liquid crystal display panel. Hence, with the presence of the patterned overcoat layer 160, the pixel electrode 170 can cover a portion of the upper part of the data line 140 to increase the display aperture ratio.
Using the above five photomasking processes can effectively raise the display aperture ratio of the liquid crystal display panel, and control the planarization of the active matrix array structure 100. However, with the application of five photomasks, the costing down of the manufacturing process becomes difficult. Hence, in order to be cost effective, it is important to decrease the number of photomasks being used.